The use of high speed switch mode DC/DC converters (switchers) for voltage conversion and regulation in many electronic systems and products has become attractive for reasons including efficiency, power dissipation, thermal management, converter size and weight, battery life, and tight voltage regulation. Gradually switchers have replaced linear regulators in a wide range of applications. The operation of a switch mode power supply is well known in the art, involving the continual switching of current into an inductor-capacitor low pass filter network, whereby feedback is used to control the energy storage rate in an inductor to match load power consumption and thereby to maintain a constant output voltage.
One fault scenario, which must be considered in any switch mode converter's design, is sustained operation in the presence of a shorted load. The impact of a shorted load depends heavily on which converter topology is used. For example FIG. 1 shows a synchronous "buck" converter 10 comprising an input voltage source Vbatt; a low pass filter network 12 having an inductor 14 and a filter capacitor 16; a high-side solid state switch 20; a low-side solid-state switch 22; and an output voltage Vout, which drives a load 25. Buck converter 10 steps down input voltage Vbatt to a lower output voltage Vout. High-side switch 20 can be shut off permanently whenever a shorted load is present, thereby disconnecting input voltage Vbatt from load 25.
A "boost" converter 30, shown schematically in FIG. 2, comprises an inductor 38 connected at one end to an energy source Vbatt and at the other end to a low-side switch 40 and a rectifier diode 34 having a cathode 36, which in turn is connected to a filter capacitor 42 and to a load 32. Current from energized inductor 38 flows through forward-biased rectifier diode 34 to charge filter capacitor 42 and to drive load 32 at an output voltage Vout greater than or equal to Vbatt. The inductor remains inductive (i.e. reactive) as long as its current does not build up to a sufficiently large value that it saturates, whereby it behaves purely resistively, allowing the current to rise rapidly. The current in the inductor is therefore controlled by the on-time of switch 40.
With "boost" converter 30, in the case of a shorted load 32, rectifier diode 34 has a grounded cathode 36, meaning that rectifier diode 34 is always forward biased. This means that the current in inductor 38 is no longer controllable by low-side switch 40. Inductor 38 then saturates, and current eventually builds up to a steady-state value Vbatt/R, where R is the sum of the resistances of the rectifier diode's and inductor's ESR (equivalent series resistance) and any resistance in the "short" itself. Destruction of rectifier diode 34 due to overcurrent heating generally results.
A similar problem occurs in the synchronous boost converter 50 of FIG. 3. A power MOSFET 52 is a synchronous rectifier that replaces a rectifier diode 34 in many power converter circuits. The motivation is primarily one of improved efficiency due to lower conductive- or on-state voltage drop. Another benefit is the capability for high speed operation, since no charge is stored as minority carriers during conduction.
In synchronous boost converter 50, power MOSFET 52 (either N or P-channel) is driven into a low-resistance conductive- or on-state when a low-side N-channel switch 54 is turned off, and the voltage at inductor 38 forces the voltage of an isolated node 58 above the output voltage Vout. Since a normal power MOSFET 52 has an intrinsic antiparallel parasitic diode 60 across its source-to-drain terminals 62, 66, it must be connected in the same orientation as the rectifier diode 34 that it replaces, to prevent loading of output voltage Vout by low-side N-channel switch 54 during normal converter operation. In a boost converter, where output voltage is generally higher than input voltage, this means that the cathode end of parasitic diode 60 is connected to the output terminal, and the anode end to the isolated node 58 connected to inductor 38. In the presence of parasitic diode 60, any shorted load results in unavoidable PN diode current, because the diode's cathode voltage is pulled more negative than its anode voltage. Only by effectively eliminating parasitic diode 60 can the short circuit overcurrent be avoided. In normal operation, however, parasitic diode 60 is beneficial, since it provides a path for the inductor current before MOSFET gate 72 can be turned on (the so-called break-before-make interval typically lasting for tens to hundreds of nanoseconds).
Parasitic diode 60 is a consequence of a sourcebody short 68 inherent in power MOSFETs. By shorting source 62 to the body 64 of power MOSFET 52, the drain 66 to body 64 parasitic diode 60 becomes connected across drain 66 to source 62 terminals of power MOSFET 52 (see FIGS. 4 and 5). In the case of a shorted load 70, parasitic diode 60 becomes forward biased and conducting, even if the power MOSFET gate 72 is turned off. Eventually current rises and power MOSFET 52 is destroyed thermally. The problem occurs for either N-channel or P-channel synchronous rectifier outputs.
Source-body short 68 is shown, for example, in a vertical MOSFET structure 80 illustrated in crosssection in FIG. 4, and shown schematically in FIG. 5. FIG. 4 shows an N+ source 82, a P+ body contact region 84, a P-body 86, which forms the MOSFET structure 80 formed in an N-epitaxial region 88. N-epitaxial region 88 and an N+ substrate 89 form the drain of MOSFET 80. The parasitic body-to-drain diode 60 is shown schematically in FIG. 5. A parasitic source-to-body diode 90 is shorted by the source-to-body short 68. In a normal power MOSFET this results in a diode conducting only in the source-to-drain direction. That is true whether it is a trench MOSFET, a lateral or quasi-vertical MOSFET, or another type of vertical power MOSFET.
What is needed is a means to disconnect a load from an inductor or energy source of a switch mode DC/DC boost converter, that does not increase the on-state power loss in a given size rectifier diode or synchronous rectifier.